In recent years there has been an increase in interest in the utilization of thermoplastics in fabricating fiber-reinforced composites. Thermoplastic matrices offer the possibility of reducing the fabrication costs of advanced structures. The cost reductions are possible because faster and less expensive manufacturing procedures can be followed as compared to those employed with conventional resin matrices.
A problem associated with thermoplastic composites relates to the high fabrication temperatures that are required. Temperatures in excess of 200.degree.-300.degree. F. above the glass transition temperature (Tg) of the thermoplastic material are usually necessary to sufficiently decrease the bulk viscosity for the thermoforming process. The problem becomes more critical as the use temprature of the structural composite is extended since higher use temperatures require thermoplastics with higher glass transition temperatures and consequently higher fabrication temperatures. The disadvantages of using very high processing temperatures (800.degree.-1000.degree. F.) include the unfavorable economics of high temperature tooling and the danger of thermal degradation of the polymer during fabrication.
The use of thermoplastic resins as structural material in aircraft has also been limited because of problems associated with linear polymeric systems. Linear polymers have very low solvent, creep and craze resistance. Solvents normally found around an aircraft or air field, such as hydraulic fluids, brake fluids, paint strippers, and the like, are potential hazards to such systems.
From the foregoing it is seen that the ideal thermoplastic material should meet certain requirements. Thus, the material should be one having a low glass transition temperature for favorable processing, and during fabrication its glass transition temperature should increase so as to extend its use temperature. The thermoplastic material should also have the capability to become lightly crosslinked when thermally treated during fabrication so as to diminish the various solvent induced problems inherent in linear polymeric materials.
It is an object of this invention, therefore, to provide high molecular weight thermoplastic material with a glass transition temperature that advances on thermal treatment.
Another object of the invention is to provide thermoplastic material which lightly crosslinks on thermal treatment.
Other objects and advantages of the invention will become apparent to those skilled in the art upon consideration of the accompanying disclosure.